Complexation of vanadium with amidoxime and carboxyl groups: uncovering the competitive role of vanadium in uranium extraction from seawater

At present, amidoxime-based adsorbents are considered to be the most promising materials for extraction of uranium from seawater. However, the high concentrations of transition metals especially vanadium strongly compete with uranium in the sequestration process, which is extremely limited the commercial use of amidoxime-based adsorbents. In this work, the coordination modes, bonding nature, and stabilities of possible vanadium(IV) (VO2+) and (V) (VO2+, VO3+, V5+) complexes with amidoximate (AO(-)), carboxyl (Ac-), glutarimidedioximate (HA(-)) and deprotonated glutarimidedioximate (A(2-)) on single and double alkyl chains (R = C-13 H-26) are systematically explored by quantum chemical calculations. Different from the uranyl (UO22+) complexes, the AO - groups of the vanadium(IV) and (V) complexes prefer to coordinate as monodentate and chelate ligands, while few species with AO - groups in eta(2)-binding mode have been observed in the vanadium complexes. Besides, the vanadium complexes are predicted to have obvious covalent metal-ligand bonds. According to thermodynamic stability analysis, all the vanadium complexes with AO(-), Ac-, HA(-) and A(2-) ligands on double alkyl chains are found to be more stable than corresponding complexes with ligands on a single chain. The synergistic effect of the amidoxime and carboxyl groups can be observed in most of VO2+ and VO3+ complexes with mixed ligands (AO(-)/Ac-). The vanadium(IV) and (V) complexes are more stable than the corresponding uranyl complexes, and the adsorption capability of the amidoxime-based adsorbents toward vanadium(V) ions decrease in the order of VO2+ > VO3+ > V5+. The dioxovanadium cation VO2+ is predicted to form multinuclear vanadium complex in the sequestration process, possibly resulting in higher stable VO2+ complexes. Therefore, the higher complexation ability of the amidoxime-based adsorbents toward vanadium over uranium is probably due to the differences in the coordination modes and bonding nature. The current results might provide important clues for rational design of efficient ligands in sequestration of uranium from seawater.